Black Friday: lifetime access is on a special offer in the app, until November 30. Black Friday: special offer on lifetime access, until November 30. Download
마찰기어는 기계의 효율성을 높이는 데 사용됩니다.machalgi-eoneun gigyeui hyoyulseong-eul nopineun de sayongdoemnida.Friction gears are used to increase the efficiency of machines.